In a paper entitled “SELF ALIGNED VERTICAL MIRRORS AND V-GROOVES APPLIED TO A SELF-LATCHING MATRIX SWITCH FOR OPTICAL NETWORKS” presented by Philippe Helin, et al. in Thirteenth IEEE International Micro Electro Mechanical Systems Conference (MEMS-2000) held on Jan. 23 through 27, 2000 at Miyazaki, Japan, for example, a technology is described that if an orientation-dependent wet etching or chemical anisotropic wet etching is applied to a single crystal silicon substrate the surface of which is (100) crystal face, a thin board-like or sheet-like mirror that erects from and is integral with the etched silicon substrate and four V-shaped grooves that extend on the silicon substrate along two straight lines orthogonal with each other and passing through the center of the mirror can be fabricated at the same time. In addition, there is another description in this paper that when a mirror erecting from a silicon substrate the surface of which is (100) crystal face is fabricated by applying a chemical anisotropic wet etching to the silicon substrate, the mirror surface of the mirror becomes (100) crystal face so that the accuracy in verticality and flatness of the mirror surface comes to much high, and therefore, the optical loss can be kept to a minimum.
In the optical switch disclosed in the above paper, four optical fibers are mounted in the four V-shaped grooves formed at angular intervals of 90°, respectively, and the optical switch operates such that light signals emitted respectively from two output side optical fibers are switched by the thin board-like mirror so that either one of them is entered into corresponding one input side optical fiber. For example, the optical switch operates such that in case the mirror is situated in the optical path, a light signal emitted from one of the two output side optical fibers disposed at adjacent positions is reflected at right angles to the incident light signal by the front mirror surface or rear mirror surface of the mirror and is entered into corresponding one of the input side optical fibers, and in case the mirror is not situated in the optical path, a light signal emitted from the other output side optical fiber is directly entered into the corresponding one input side optical fiber that is opposed to the other output side optical fiber.
With the manufacturing method described in the above paper, the thin board-like mirror can be formed accurately at a position that the mirror surfaces (front vertical surface and rear vertical surface) thereof form accurately an angle of 45° with the four V-shaped grooves, respectively. However, the thickness of the mirror varies with an error in the etching time so that the thickness thereof can become thicker or thinner than a predetermined thickness, which results in a displacement in position of the mirror surfaces. If the position of the mirror surface should be displaced, the axis of the optical path also deviates so that it is difficult to enter light signals emitted from the two output side optical fibers into corresponding two input side optical fibers at the same time with low optical loss. In other words, in case a 2×2 optical switch is constructed by use of one thin board-like mirror, there occurs a problem that the axis of the optical path deviates.
Further, in FIG. 2 of Japanese Patent Application No. 2000-270621 (filed on Sep. 6, 2000) previously proposed by the present applicant was shown a prior art optical switch having substantially the same construction as that of the optical switch described in the above paper, and defects of this optical switch due to the thickness of a mirror were described with reference to FIG. 3 thereof. The details thereof should be referred to Japanese Patent Application No. 2000-270621.
In the optical switch described in the above paper, in order to make the thickness of the mirror a predetermined thickness, the etching time in applying an anisotropic wet etching to a silicon substrate must be controlled with high precision. For this reason, there is a drawback that the working efficiency is very low.